fix: use DiskUsage instead of VolumeList to populate Volume.UsageData

[glav-git]

Fixes #450

Explanation:

The renderer of volumes list already had support for displaying Volume.UsageData:

lazydocker/pkg/gui/volumes_panel.go

Lines 80 to 83 in 577797d

	if volume.Volume.UsageData != nil {
	output += utils.WithPadding("RefCount: ", padding) + fmt.Sprintf("%d", volume.Volume.UsageData.RefCount) + "\n"
	output += utils.WithPadding("Size: ", padding) + utils.FormatBinaryBytes(int(volume.Volume.UsageData.Size)) + "\n"
	}

But it was never displayed, since according to Volume.UsageData field comment it was available only when using /system/df endpoint:

lazydocker/vendor/github.com/docker/docker/api/types/volume/volume.go

Lines 56 to 75 in 577797d

	// UsageData Usage details about the volume. This information is used by the
	// `GET /system/df` endpoint, and omitted in other endpoints.
	//
	// swagger:model UsageData
	type UsageData struct {
	// The number of containers referencing this volume. This field
	// is set to `-1` if the reference-count is not available.
	//
	// Required: true
	RefCount int64 `json:"RefCount"`
	// Amount of disk space used by the volume (in bytes). This information
	// is only available for volumes created with the `"local"` volume
	// driver. For volumes created with other volume drivers, this field
	// is set to `-1` ("not available")
	//
	// Required: true
	Size int64 `json:"Size"`
	}

Since DiskUsage returns literally the same Volume type as VolumeList - I was able to simply swap method calls (and add option to fetch only volumes data) and was proud to finally see Size and RefCount of my docker volume.

Before

After